The present invention relates to an electrical machine, in particular to an alternator with a multiple-phase stator winding.
With alternators for motor vehicles, electrical machines are primarily used that include a claw-pole rotor excited by direct current, in order to adequately supply the DC electrical system of the particular motor vehicle even while the engine is idling. In addition to numerous other requirements on the alternator, it is also necessary to dampen the magnetic noise of the alternator, which is noticeable—and disturbing—in the lower rotational speed range of the machine in particular. To suppress the magnetic noises, it is known to distribute the individual phase windings on the three-phase stator winding of the alternator in such a manner that they are placed partially in the slots of the adjacent phase winding. This measure results in reduced alternator output and increased losses, however. Due to the ripple of the direct current that is output, it is also possible for oscillatory noises to occur in the wiring harnesses of the motor vehicle at certain engine speeds.
It is also known to equip the alternator with a six-phase system, in order to double the frequency of the rectification and, therefore, to reduce the ripple of the direct current, which is supplied via a rectifier assembly to a storage battery of the motor vehicle electrical system. It is known from EP 0454 039 B1 (FIG. 6) to design the stator winding of an alternator using two winding systems, each of which includes three phase windings that are interconnected to form a star connection. The phase windings are offset electrically by 120° in the star connection. The two winding systems are electrically offset from each other by approximately 30°. The magnetic noises of the machine that are produced are not adequately damped, however, particularly in the lower rotational speed range. A further disadvantage is the fact that machines of this type still have a large voltage and torque ripple, which applies in particular for high-output machines when operated either in the alternator mode or the engine mode. Finally, it is known from DE 102 09 054 A1 to use a seven-phase stator winding in order to dampen the magnetic noises and to reduce the current ripple of a motor vehicle alternator, and to connect its seven phase conductors—which lie side-by-side in the slots of a stator core—in a star connection, or to connect them in series to form a heptagon. Even though these designs have a lower current ripple and less magnetic noise than do seven-phase systems, they are still inadequately damped in the lower rotational speed range. In addition, the upward slope of the performance characteristic of the machine is insufficient, since, in motor vehicles, the power requirement in the vehicle electrical system is often high in the idling range of the internal combustion engine.